1. Field of the Invention
The present invention relates to a magneto-resistive element, particularly to a magneto-resistive element that is used in a thin-film magnetic head of a magnetic recorder, such as a hard disk drive.
2. Description of the Related Art
Magnetic heads that uses a GMR (Giant Magneto-resistive) element for reading magnetic data have been used in order to meet the need for higher magnetic recording densities. In particular, a GMR element using a spin valve (SV) film has the potential for providing a highly sensitive magnetic head, because it exhibits a large change in electric resistance for sense current, which is applied to the element in order to read data that are recorded in a recording medium. In this specification, SV film means an assembly of stacked layers that has a ferromagnetic layer whose magnetization direction is fixed in one direction (hereafter referred to as a pinned layer), a ferromagnetic layer whose magnetization direction is changed in accordance with an external magnetic field that is generated by a recording medium (hereafter referred to as a free layer), and a nonmagnetic intermediate layer that is sandwiched between the pinned layer and the free layer. In an SV film, the magnetization direction of the free layer makes a relative angle with magnetization direction of the pinned layer in accordance with an external magnetic field, and magnetic resistance is changed depending on the change in the spin-dependent scattering of conduction electrons that is caused in accordance with the relative angle. A magnetic head detects the change in magnetic resistance, and reads magnetic information in the recording medium.
A CIP (Current in Plane)-GMR element, in which sense current is applied in parallel with layer surfaces, has been conventionally used as a MR element that uses an SV film. However, a magnetic head using a CPP (Current Perpendicular to the Plane)-GMR element, in which sense current is applied in a direction that is perpendicular to layer surfaces, has been developed recently in order to meet the need for higher magnetic recording densities. A CPP-GMR element exhibits a smaller resistance than a TMR (Tunnel Magneto-resistance) element that uses a TMR film, which is another CPP type element. A CPP-GMR also exhibits a higher output than a CIP-GMR element even when magnetic information which is recorded in a narrow track width is read. Therefore, the CPP-GMR element is expected to be a promising element having high potential.
However, if an SV film having the same layer structure as used in a CIP-GMR element is applied to the CPP-GMR element without being modified, then it is difficult to obtain a sufficient change in magnetic resistance. This is mainly because the electric resistance in layers that contribute to the change in resistance (the free layer, the pinned layer, and the nonmagnetic intermediate layer) occupies a small percentage relative to the total resistance of the element. Specifically, in a CIP-GMR element, a sufficient amount of change in magnetic resistance is obtained due to spin-dependent scattering that is caused in an in-plane direction because electric current is applied in the direction that is parallel to the layer surfaces. However, in a CPP-GMR element, spin-dependent scattering is not sufficiently caused at the boundaries between layers because sense current is applied in the direction that is perpendicular to the layer surfaces. Further, in an ordinary GMR element, boundaries that contribute to spin-dependent scattering are limited to two, i.e., the boundary between the nonmagnetic intermediate layer and the free layer, and the boundary between the nonmagnetic intermediate layer and the pinned layer. Accordingly, boundaries do not contribute sufficiently to the change in magnetic resistance as a whole. On the other hand, a CPP-GMR element generally exhibits a larger amount of bulk scattering, which is scattering of conduction electrons within layers, than a CIP-GMR element, because sense current flows through each layer. Therefore, bulk scattering contributes more to the change in magnetic resistance in a CPP-GMR element. For this reason, in a CPP-GMR element, a useful effect can be obtained by increasing the thickness of the free layer and the pinned layer in order to achieve a large amount of change in magnetic resistance.
In Japanese Patent Laid-Open Publication No. 2003-152239, a technique is disclosed to increase the number of boundaries by inserting a nonmagnetic intermediate layer in the free layer or in the pinned layer, and thereby to enhance the magneto-resistance effect, instead of increasing the thickness of the free layer or the pinned layer. A free layer is proposed in this patent document in which a CoFeB alloy layer, a Cu nonmagnetic layer, and a CoFeB alloy layer are stacked (CoFeB/Cu/CoFeB layers). Since a large amount of spin polarization is caused at the boundaries between the CoFe base alloy layer and the Cu layer and, as a result, spin-dependent scattering is promoted, a large change in magnetic resistance is obtained.
In Japanese Patent Laid-Open Publication No. 2003-133614, a layer structure of a SV film is disclosed in which at least either the free layer or the pinned layer has a ternary alloy, which consists of nickel, iron, and cobalt, in order to obtain a large change of magnetic resistance.
As described above, in a CPP-GMR element, a large magneto-resistance ratio can be effectively obtained by increasing the thickness of the free layer and the pinned layer, and thereby enhancing the contribution of bulk scattering. However, an increase in the thickness of a SV film leads to an increase in the distance between shield layers. This is not preferable for obtaining a high linear recording density. Accordingly, it is desirable to obtain a large magneto-resistance ratio without increasing the thickness of the free layer because the thickness of the SV film is limited. In addition, the free layer needs to have proper magnetic properties that are required for a soft magnetic metal (referred to as soft magnetic properties), as well as a large magneto-resistance ratio, when it is used for the read element of a magnetic head. The soft magnetic properties mean low coercive force and low magneto-striction.